a. Field of the Invention
The present invention relates to fixing apparatus for fixing fused toner image on a support such as a sheet of paper by heating the toner image.
B. Description of the Prior Art
Heretofore, it is well known to produce a toner image on a suitable support according to the electrophotography method, electrostatic printing method or magnetic printing method and then to fix the formed toner image if necessary.
The toner generally used for this purpose is formed of fine particles having a particle size in the range of 0.1 - 50.mu. and is made by mixing thermoplastic resin and coloring agent. Using such a toner, a toner image is formed on a support according to various dry- or wet developing methods. Thereafter, the formed toner image may be fused or melted and permanently fixed on the support by using heat, pressure, solvent steam or the like.
The fixing method using solvent steam is more efficient than the method using heat. But, the former has a hygienic problem caused by the scattering of stinking solvent steam.
The fixing method using pressure which is called pressure fixing method has some advantages. In the first place, it alows fixing with a small amount of energy. In the second place, an instant speed-up is possible. But, the manufacture of pressure sensitive toner is very complicated and expensive. This important drawback has prevented the method from being widely accepted and until now its use has been limited.
For the above reason, the heat fixing method has been employed most widely to fix toner image. As a heat fixing method, there are known and used two types of fixing processes. One is of the type in which the support is passed through the nip area between a pair of two heating rollers where the toner image is compressed and heated with transmission heat transmitted from the heating rollers. Another is of the type in which the support is heated with radiant heat from an infrared lamp or the like.
The former, that is, the heat fixing process using a heating roller (hereinafter called "heat roller fixing process") is acknowledged to have a better thermal efficiency, compared with the other heat fixing process using radiant heat or heat chamber.
For the heat roller fixing process, factors that will affect the fixation are temperature, pressure and contact width (nip) of the two heating rollers.
The temperature of heating rollers can not exceed a certain upper limit. The heating roller is usually composed of a heat resisting rubber. In addition, in order to prevent the toner from being offset, the surface of the heating roller is coated with liquid surface lubricant. Therefore, if the heating rollers are heated up to a too high temperature, there will arise troubles of heat deterioration of rubber and bonding agent and evaporation of the surface lubricant. As usual, the upper limit for the heating roller's surface temperature is about 200.degree. C. The heating rollers can not be heated to a temperature over this upper limit for the reason mentioned above.
The heating value or calorific value transmitted from the heating roller to the toner and support is represented by the following equation:
.differential.Q/.differential.t=A.multidot.K.multidot.(.differential.T/.dif ferential.X)
wherein Q is heating value (cal) transmitted from the heating roller to the toner and support, t is the time (sec), A is the area of surface contact (cm.sup.2), K is the heat transfer coefficient (cal/.degree. K.cm.sup.2 sec), T is temperature difference between two heat transmissive bodies and X is the distance from the heating roller surface (cm).
So, (.differential.Q/.differential.t) represents the transmitted heating value (cal/sec) per unit time and (.differential.T/.differential.X) represents the temperature gradient (.degree. K./cm). The equation means that when the difference in temperature between the heating roller surface and support including toner is large, the velocity of heat transmisson becomes high. In other words, the velocity of heat transmission or convection is high at the beginning of contact and thereafter it gradually drops. Therefore, at the beginning of a fixing step, heat is rapidly transmitted from the heating rollers to support owing to the existing large temperature difference. However, with the rise of temperature of the toner, the heat transmission rate becomes remarkedly smaller. Due to this fact, the fixing speed attainable by the heat roller fixing process is relatively low, which is an important drawback of this fixing process.
In order to attain a high speed fixation by heating rollers, it is necessary to keep a steep temperature gradient by raising the heating surface temperature up to a point far higher than the fusing point of the toner or to extend the contact time by bringing the two rollers into contact with the highest possible pressure so as to widen the nip width accordingly.
However, as mentioned above, heating the rollers to high temperature may cause a problem. In practice, it is not allowed to raise the roller surface temperature up to a point far higher than the fusing point of the toner which is generally in the range of from 100.degree. C. to 150.degree. C. On the other hand, the high pressure contact of a pair of heating rollers may cause the deformation and deterioration of the rubber material at the surface portion of the heating rollers. Since the heat deprived of by the support becomes large, the thermal efficiency may be reduced accordingly. Also there may occur some other troubles such as formation of creases on the support, vagueness of produced image and mechanical instability of the apparatus.
In the case of a radiant heat fixing process, the heating value given to the toner by radiant rays is represented by the following equation:
QR=A.multidot..alpha..multidot.ER.multidot.t
wherein QR is the heating value given to the toner (including the support) by radiant rays, .alpha. is the absorption heat conversion efficiency, A is the area of irradiation (cm.sup.2), ER is the radiant value (cal/cm.sup.2 sec) and t is the irradiation time. As will be understood from the above equation, the heating value given to toner by radiant rays is proportional to the radiant ray value and irradiation time. The temperature rising speed hardly depends upon toner temperature, which is different from the case of the heat roller fixing process. Therefore, the advantage of a rapid conversion of absorbed radiant rays to heat can be obtained. In view of these facts, it may be considered that the radiant heat fixing process is preferable for a rapid fixation. But, in practice, its thermal efficiency is very low and it is said that the portion of radiant rays practically used for fixing is only 20% at the most. This is because the heat transmission efficiency of the toner layer is extremely poor. As known to those skilled in the art, fixing is completed by the fusion of toner particles and their adhesion to each other as well as to the support. It is necessary for fixing to have heat transferred from the raised temperature portion of the toner surface layer to the support surface and to keep a sufficient temperature on the intersurface between the toner and support. An unfixed toner layer (voids: about 50%) in the conventional radiant heat fixing process contains a large amount of air and, therefore, its heat transfer rate is only about 1/2-2/3 of the rate of a compressed toner layer (voids: 10-20%). In addition, the thickness of the unfixed toner layer is larger than that of the compressed toner layer. For these reasons, the heat transmission efficiency of the toner layer is very poor. As other reasons for the low heat transmission efficiency,, the following facts may be pointed out.
Absorption of radiant thermal rays occurs only at a limited area near the surface of the toner layer, which causes the temperature to rise only at the surface portion of the toner layer. Since the radiant ray irradiated to the toner layer hardly reaches the inside of the layer, a long time is required to completely heat and melt the whole toner layer.
Although toner has a relatively high absorption factor of radiant thermal rays, the support which is the background portion of image shows a low absorption rate. Due to it, the heat absorbed by the toner portions is dispersed out in the support and the rise of temperature at the toner portion is delayed.
In summary, the low thermal efficiency in the radiant heat fixing process is due to the fact that the area of radiant ray absorption of the toner image portion is too small and that the radiant rays which the toner image portion has absorbed does not adequately contribute to fixation.